Bobbing revolver hammer

S&W Mod 66-4 using Federal primers Question, given the same mainspring tension, is a more positive primer ignition achieved by a bobbed hammer perhaps travelling slightly faster than a standard hammer travelling slightly slower.Tony

Given those two choices, as asked, I don't think there would be much difference, as both should have power to spare when it comes to setting-off the primers.

Years ago, I shot Police Pistol Competition (PPC, also called Police Pistol Combat), and there were many gunsmiths that would excessively lighten the hammer to get a very light double-action trigger pull weight on PPC guns. They were successful, but many of these finely-tuned competition guns came with special instructions to make sure they would work properly and reliably. Some were actually tuned to a certain brand of primer (Federal), and would not function reliably with other brands! Others had to be kept very clean and lubed regularly so the super-light hammer would move fast enough to hit the primer with the required force. The difference between these guns and a stock gun was often described as "slapping" the primer, versus "crushing" it.

In a K-frame S&W like your M66, I think removing the hammer spur still leaves plenty of mass to crush the primer, but reducing the mainspring pressure (loosening the strain screw) or using an aftermarket-brand mainspring in conjunction with a spurless hammer might cause problems. Also, if the gun is to be used for defense, I'd recommend trying it out with the EXACT ammunition you will carry in it, and I'd test it in the coldest conditions it might be subjected to, just to make sure the combo is reliable (cold temps can thicken some oils/lubes, slowing hammer-fall and causing misfires).

I've bobbed several of my K-frames over the years (and one N-frame), and never had any problems with them. The J-frames, on the other hand, can be touchy...

DJ is right on.Federal is the "softest" primer to indent,that's why they are recommended for lightly tuned revolvers.A stock gun isn't going to lose enough mass to make a difference.An old friend of mine that shot a K frame in revolver class pins and combat had it so light you could litterally use your pinky DA,but it was almost guarranteed one out of 50 wouldn't pop a Federal primer.

Bobbing the hammer on a revolver is not generally done to get a lighter trigger pull. It is usually done to make a concealed carry gun, which will not be fired in single-single action mode, less likely to snag when drawn. There has been a lot of discussion over the years about if or if not bobbing the hammer would make the gun more likely to miss-fires. Long story short, those who are knowledgeable in the discipline of physics maintain that lightening the hammer (removing the spur), increases the speed in hammer fall and in fact make the lightened hammers more reliable in firing the primers. What most often is the case where a person bobs the hammer and gets misfires is that bobbing the hammer was not the only thing they did...they put in a lighter mainspring as well as other work on the action. Then when the primer does not go bang, they say, "Whoops! The bobbed hammer is making light primer strikes."
On a strictly defensive double-action revolver, I have not seen one convincing argument for not bobbing the hammer. There are however, fanciful, unlikely scenarios than the more creative among us can come up with. Like, saving a hostage. For police that could be a consideration, but I would not want a mall-ninja shooting a perp holding me at gun-point. There there is the never-explained, "for a long shot". That one is never really defined. But it does beg the question: What long shot would be required from a concealed permit holder? Should he not be taking cover or putting distance between the danger and himself? Here in Michigan, concealed weapon permits usually come with the training that explains that using deadly force must be last resort, totally defensive and that you will have to defend your actions legal-wise if you fire your gun. I am sure however, there will be some creative input from someone's unlikely fantasy.
In short, I carry one of two .38 Spl. snub-guns. One a Taurus M85 that came without a spur, and the other, a S&W 36 that I have bobbed. They are pocket carry guns, and both are 100% reliable with zero misfires despite the fact that I have lightened the M36...the Taurus, believe it or not, needed no lightening or modifications whatsoever. I also have a 686 sans hammer spur for double-action shooting-for-play. That one is also 100% reliable.
It is notable that there are some models of double-action revolvers that come from the factory with no hammer spur. I doubt that they would do that if it impaired reliability, and I have no reason to believe that they put anything but their standard hammer springs in those guns.